1. Field of the Invention
The present invention relates to the use of LEDs in horticultural lighting applications. In particular, the present concerns a lighting fixture for facilitating plant growth comprising at least one Light Emitting Diode (LED) having spectral characteristics including a peak in the wavelength range from 600 to 700 nm. The present invention also concerns novel light emitting components which are particularly suitable for facilitating plant growth and comprising a light emitting compound semiconductor chip.
2. Description of Related Art
On the Earth the sun is the main source of visible (i.e. light) and invisible electromagnetic radiation and the main factor responsible for the existence of life. The net daily average solar energy reaching the Earth is approximately 28×10^23 J (i.e. 265 EBtu). This value is 5500 times higher than the world's annual primary energy consumption, estimated in 2007 to be 479 PBtu. The spectral distribution of the sun's radiation, as it can be measured at the earth's surface, has a broad wavelength band of between around 300 nm and 1000 nm.
However, only 50% of the radiation reaching the surface is photosynthetically active radiation (PAR). PAR, according to the CIE (Commission Internationale de L′Eclairage) recommendations comprises the wavelength region of between 400 nm and 700 nm of the electromagnetic spectrum. The laws of photochemistry can generally express the way that plants harvest radiation. The dual character of radiation makes it behave as an electromagnetic wave when propagating in space and as particles (i.e. photon or quantum of radiant energy) when interacting with matter. The photoreceptors are the active elements existing mainly on plant's leaves responsible for the photon capture and for conversion of its energy into chemical energy.
Due to the photochemical nature of photosynthesis, the photosynthetic rate, which represents the amount of O2 evolution or the amount of CO2 fixation per unit time, correlates well with the number of photons falling per unit area per second on a leaf surface. Therefore, the recommended quantities for PAR are based on the quantum system and are expressed using the number of moles (mol) or micromoles (μmol) of photons. The recommended term to report and quantify instantaneous measurements of PAR is the photosynthetic photon flux density (PPM), and is typically expressed in μmoles/m2/s. This gives the number of moles of photons falling at a surface per unit area per unit time. The term photosynthetic photon flux (PPF) is also frequently used to refer to the same quantity.
Photoreceptors existing in living organisms such as plants use the radiant energy captured to mediate important biologic processes. This mediation or interaction can take place in a variety of ways. Photosynthesis together with photoperiodism, phototropism and photomorphogenesis are the four representative processes related to interaction between radiation and plants. The following expression shows the simplified chemical equation of photosynthesis:6H2O+6CO2(+photon energy)→C6H12O6+6O2 
As will appear from the equation, carbohydrates, such as sugar glucose (C6H2O6), and oxygen (O2), are the main products of the photosynthesis process. These are synthesized from carbon dioxide (CO2) and water (H2O) using the energy of the photons harnessed by using specialised photoreceptors such as chlorophylls and converted into chemical energy.
Through photosynthesis, the radiant energy is also used as the primary source of chemical energy, which is important for the growth and development of plants. Naturally, the input—output reactant balance of the equation is also dependent on the quantity (i.e. number of photons) and quality (i.e. energy of the photons) of the radiant energy and, consequently, also of the produced biomass of the plants. “Photoperiodism” refers to the ability that plants have to sense and measure the periodicity of radiation, phototropism to the growth movement of the plant towards and away from the radiation, and photomorphogenesis to the change in form in response to the quality and quantity of radiation.
The typical absorption spectra of the most common photosynthetic and photo-morphogenetic photoreceptors, such as chlorophyll a, chlorophyll b and betacarotene, and the two interconvertable forms of phytochromes (Pfr and Pr) are presented in FIG. 1.
The photomorphogenetic responses, contrary to photosynthesis, can be achieved with extremely low light quantities. The different types of photosynthetic and photo-morphogenetic photoreceptors can be grouped in at least three known photosystems: photosynthetic, phytochrome and cryptochrome or blue/UV-A (ultraviolet-A).
In the photosynthetic photosystem, the existing pigments are chlorophylls and carotenoids. Chlorophylls are located in the chloroplasts' thylakoids located in the leaf mesophyll cells of plants. The quantity or the energy of the radiation is the most significant aspect, since the activity of those pigments is closely related to the light harvest. The two most important absorption peaks of chlorophyll are located in the red and blue regions from 625 to 675 nm and from 425 to 475 nm, respectively. Additionally, there are also other localized peaks at near-UV (300-400 nm) and in the far-red region (700-800 nm). Carotenoids such as xanthophylls and carotenes are located in the chromoplast plastid organelles on plant cells and absorb mainly in the blue region.
The phytochrome photosystem includes the two interconvertable forms of phytochromes, Pr and Pfr, which have their sensitivity peaks in the red at 660 nm and in the far-red at 730 nm, respectively. Photomorphogenetic responses mediated by phytochromes are usually related to the sensing of the light quality through the red (R) to far-red (FR) ratio (R/FR). The importance of phytochromes can be evaluated by the different physiological responses where they are involved, such as leaf expansion, neighbour perception, shade avoidance, stem elongation, seed germination and flowering induction. Although shade-avoidance response is usually controlled by phytochromes through the sensing of R/FR ratio, the blue-light and PAR level is also involved in the related adaptive morphological responses.
Blue- and UV-A (ultraviolet A)-sensitive photoreceptors are found in the cryptochrome photosystem. Blue light absorbing pigments include both cryptochrome and phototropins. They are involved in several different tasks, such as monitoring the quality, quantity, direction and periodicity of the light. The different groups of blue- and UV-A-sensitive photoreceptors mediate important morphological responses such as endogenous rhythms, organ orientation, stem elongation and stomatal opening, germination, leaf expansion, root growth and phototropism. Phototropins regulate the pigment content and the positioning of photosynthetic organs and organelles in order to optimize the light harvest and photoinhibition. As with exposure to continuous far-red radiation, blue light also promotes flowering through the mediation of cryptochromes photoreceptors. Moreover, blue-light-sensitive photoreceptors (e.g. flavins and carotenoids) are also sensitive to the near-ultraviolet radiation, where a localized sensitivity peak can be found at around 370 nm Cryptochromes are not only common to all plant species. Cryptochromes mediate a variety of light responses, including the entrainment of the circadian rhythms in flowering plants such as the Arabidopsis. Although radiation of wavelengths below 300 nm can be highly harmful to the chemical bonds of molecules and to DNA structure, plants absorb radiation in this region also. The quality of radiation within the PAR region may be important to reduce the destructive effects of UV radiation. These photoreceptors are the most investigated and therefore their role in control of photosynthesis and growth is known reasonably well. However, there is evidence of the existence of other photoreceptors, the activity of which may have an important role in mediating important physiological responses in the plant. Additionally, the interaction and the nature of interdependence between certain groups of receptors are not well understood.
Photosynthesis is perhaps one of the oldest, most common and most important biochemical process in the world. The use of artificial light to substitute or compensate the low availability of daylight is a common practice, especially in northern countries during the winter season, for production of vegetable and ornamental crops.
The time of artificial electric lighting started with the development by Thomas Edison in 1879 of Edison's bulb, commonly known today as the incandescent lamp. Due to its thermal characteristic, incandescence is characterised by a large amount of farred emission, which can reach approximately 60% of the total PAR. In spite of the developments that have taken place over more than a century, the electrical efficiency of incandescent lamps, given by the conversion efficiency between electrical energy consumed (input) and optical energy emitted (output) within the visible spectral region, is still very poor. Typically it is around 10%. Incandescent light sources suffer also low lifetime performances, typically lifetime is not longer than 1000 hours. In plant-growth applications their use is limited.
Growth of ornamental plants is one of the applications where incandescent lamps can still be used. Floral initiation can be achieved with long day responsive species using overnight exposure to low photon fluence rates using incandescent lamps. The high amount of far-red radiation emitted is used to control the photomorphogenetic responses throughout the mediation of the phytochromes.
Fluorescent lamps are more commonly utilized in plant-growth applications than incandescent lamps. The electro-optical energy conversion is more efficient in comparison to incandescent lamps. Tubular type fluorescent lamps can achieve electrical efficiency values from typically around 20% to 30%, where more than 90% of the emitted photons are inside the PAR region with typical lifetimes of around 10000 hours. However, especially designed long-lifetime fluorescent lamps can reach lifetimes of between 30000 hours. Besides their reasonable energy efficiency and lifetime, another advantage of fluorescent lamps in plant growth is the amount of blue radiation emitted. This can reach more than 10% of the total photon emission inside PAR, depending on the correlated colour temperature (CCT) of the lamp. For this reason, fluorescent lamps are frequently used for total substitution of natural daylight radiation in close growth rooms and chambers. The blue radiation emitted is indispensable to achieve a balanced morphology of most crop plants through the mediation of the cryptochrome family of photoreceptors.
The metal halide lamp belongs to the group of high-intensity discharge lamps. The emission of visible radiation is based on the luminescent effect. The inclusion of metal halides during manufacture allows to a certain extent the optimization of the spectral quality of the radiation emitted. Metal halide lamps can be used in plant growth to totally replace daylight or for partially supplementing it during the period of lower availability. The high PAR output per lamp, the relatively high percentage of blue radiation around 20% and the electrical efficiency of approximately 25%, makes metal halide lamps an option for year-round crop cultivation. Their operation times are typically 5,000 to 6,000 hours. The high-pressure sodium (HPS) lamp has been the preferred light source for year-round crop production in greenhouses. The main reasons have been the high radiant emission, low price, long life time, high PAR emission and high electrical efficiency. These factors have allowed the use of high-pressure sodium lamps as supplemental lighting sources supporting vegetative growth in a cost-effective way during wintertime in northern latitudes.
However, the spectral quality in HPSs lamps is not optimal for promoting photosynthesis and photomorphogenesis, resulting in excessive leaf and stem elongation. This is due to the unbalanced spectral emission in relation to the absorption peaks of important photosynthetic pigments such as chlorophyll a, chlorophyll b and betacarotene. The low R/FR ratio and low blue light emission in comparison with other sources induces excessive stem elongation to most of the crops grown under HPS lighting. Electrical efficiencies of high-pressure sodium lamps are typically within 30% and 40%, which make them the most energy-efficient light sources used nowadays in plant growth. Approximately 40% of the input energy is converted into photons inside the PAR region and almost 25% to 30% into far-red and infra red. The operation times of high pressure sodium lamps are in the range from about 10,000 to 24,000 hours.
The low availability of daylight in northern latitudes and the demand of consumers for quality horticultural products at affordable prices year-round set demands for new lighting and biological technologies. Also production yields globally can be significantly increased if daylight is available up 20 to 24 hours per day. Therefore, approaches that can reduce production costs, increase yields and quality of the crops are needed. Lighting is just one of the aspects involved that can be optimized. However, its importance cannot be underestimated. The increase in electricity prices and the need to reduce CO2 emissions are additional reasons to make efficient use of energy. In year-round crop production in greenhouses, the electricity cost contribution to overhead costs may reach in some crops approximately 30%.
Although existing light sources commonly used for plant growth may have electrical efficiencies close to 40%, the overall system efficiency (i.e. including losses in drivers, reflectors and optics) can be significantly lower. The spectral quality of the radiation plays an important role in the healthy growth of the crop. The conventional light sources cannot be spectrally controlled during its utilization without the inefficient and limited utilization of additional filters. Moreover, the control of the radiation quantity is also limited, reducing the possibility of versatile lighting regimes such as pulsed operation.
Therefore, and for reasons relating to the previously described aspects, the light-emitting diode and related solid-state lighting (SSL) have emerged as potentially viable and promising tools to be used in horticultural lighting. Internal quantum efficiency of LEDs is a measure for the percentage of photons generated by each electron injected into the active region. In fact, the best AlInGaP red and AlInGaN green and blue HB-LEDs can have internal quantum efficiencies better than 50%; still challenges remain to extract all generated light out of the semiconductor device and the light fixture.
In horticultural lighting the main practical advantages of LED-based light sources in relation to conventional light sources is the directionality and full controllability of the emitted radiation. LEDs do not necessarily require reflectors, as they are naturally halfisotropic emitters. LEDs as directional emitters avoid most of the losses associated with the optics. Additionally, the narrow spectral bandwidth characteristic of coloured LEDs is another important advantage in relation to conventional broad waveband light sources. The main advantage of using LEDs as photosynthetic radiation sources results from the possibility of selecting the peak wavelength emission that most closely matches the absorption peak of a selected photoreceptor. In fact, this possibility brings with it additional advantages. The efficient usage of the radiant energy by the photoreceptor on the mediation of a physiological response of the plant is one of the advantages. Another advantage is the controllability of the response by fully controlling the radiation intensity.
The advantages mentioned previously can be further extended to the luminaire level. The inventor is aware of a luminaire with a blue LED and a red LED. The spectral emission of currently coloured AlInGaN LEDs are available from UV into to the green region of the visible spectrum. Those devices can emit in the blue and UV-A region where the absorption peaks of cryptochromes and carotenoids are located.
Chlorophyll a and the red isomeric form of phytochromes (Pr) have a strong absorption peak located around 660 nm AlGaAs LEDs emit in the same region but, partially due to low market demand and outdated technology of production, they are expensive devices if compared with phosphide or even nitride-based LEDs. AlGaAs LEDs can be also used to control the far-red form of phytochromes (Pfr), which has an important absorption peak at 730 nm
AlInGaP LEDs are based on a well-established material technology with the relatively high optical and electrical performance. Typically, the characteristic spectral emission region of AlInGaP red LEDs covers the region where chlorophyll b has its absorption peak, around 640 nm. Therefore, AlInGaP LEDs are also useful in promoting photosynthesis.
The novel commercial high brightness LEDs are not suitable for greenhouse cultivation as their main emission peak lies in the range of green wavelengths extending from 500 to 600 nm and thus not responding to photosynthesis process. However, in principal according to the art a LED light to which the photosynthesis responds can be constructed combining various types of semiconductor LEDs such as AlInGaP and AlInGaN, for red and blue colors.
There are a number of problems related to the combination of individually colored LEDs. Thus, different types of semiconductor devices will age at different rates and for this reason the ratio of red colour to blue color will vary over time, resulting further in abnormalities in a plant growth process. A second major issue is that individual single color LEDs have relatively narrow spectral coverage, typically less than 25 nm, which is insufficient for providing good photosynthesis efficiency without utilizing very high number different color and individual LEDs and causing high cost of implementation.